1. Field of the Invention:
This invention relates generally to current limiting high voltage fuses and more particularly to high voltage fuses having means for shaping and controlling the arc voltage developed by the fuse during high current interruptions.
2. Description of the Prior Art:
The current limiting effect in current limiting fuses is determined primarily by the shape and magnitude of the arc voltage developed by the fuse during high current interruptions. The characteristics of the arc voltage are externally sensitive to circuit voltage, inductance, fault level and current symmetry. The controlling factors associated with the fuse include length and morphology of the element, number of parallel elements, shunting effects such as those caused by parallel indicator wires and the like, and type and grain size distribution of the sand filler.
From the interruption standpoint, the ideal arc voltage shape would be rectangular. That is, it would instantaneously rise to a fixed, predetermined level and remain there until current extinction was assured, at which time it would then drop to zero. A number of designs have been offered over the years which attempt to approach this ideal characteristic. The most successful of these was the replacement of the wire element with one consisting of a thin ribbon or strip with perforations or notches arranged along its length.
Notches are conventionally provided in the fuse element of current limiting fuses to control and limit the fuse arc voltage generated during fault current interruption. Elements of uniform cross section, such as wires, unnotched strips and the like, lack this control feature and are capable of generating arc voltages high enough to cause insulation failures in associated equipment. However, to insure adequate performance on low current clearing, or to achieve small current ratings, certain higher voltage fuses require that wire elements be employed. One problem associated with wire elements is that the arc voltage magnitude varies directly with the fault current level. More exactly, arc voltage is a function of the available fault current in the current limiting region of interest. Additionally, for similar circumstances, arc voltage is a linear function of wire element length. Because of this, a fuse design equipped with a wire element long enough to effect satisfactory low current clearing might well generate a peak arc voltage which exceeds the value permitted by standards on high current faults. In some situations, this excessive arc voltage could actually endanger the insulation of the protected system. Ironically, the high arc voltage developed in situations of this sort is not really effective in achieving high fault interruption because it typically collapses before the fault current has been sufficiently turned around.
The arc voltage peak will be somewhat reduced and extended in time duration by increasing the number of parallel elements or by incorporating an indicator wire. The classical solution, however, consists of providing wire elements which have a varying cross section. The usual approach might be to join lengths of, say, three wires of different diameters together. The overall length would be sized to satisfy the low current clearing requirements. On high currents, the peak arc voltage would be primarily determined by the length of the smallest diameter wire, which would necessarily be the first to open. As the first peak starts to collapse, the second diameter wire opens and rebuilds the voltage peak to near the first level. The collapse and rebuild repeats with the third and final diameter. It can be seen that this technique not only limits the magnitude but also spreads out the arc voltage in an approximation of the ideal rectangular shape. The ultimate form of this approach, presently used by some fuse manufacturers, consists of a wire element with a tapering diameter.
The wire element designs just discussed have certain disadvantages. The non-uniform element is not as effective on low current interruptions as in an element of one diameter. The element is difficult and costly to fabricate. Finally, even though the element is designed to open sequentially along its length, arc voltage will still vary with the available fault current, and may therefore impose an upper limit on the interrupting rating of the fuse.